Existing laser doppler anemometers have an emitting optic, a receiving optic and an evaluation unit as basic components. The emitting optic emits two laser beams which cross at a crossing point. The crossing point displays a certain spatial expansion and defines a measurement volume. The receiving optic collects the light scattered by dispersed particles located in the measurement volume. To this end, the receiving optic is to be focused on the measurement volume.
This adjustment, i.e. the crossing of the two laser beams exactly at the focal point of the receiving, optic must be carried out with extreme precision in order to obtain correct speed measurement results with the laser doppler anemometer. The evaluation unit analyzes the scattered light received by the receiving optic and originating with the dispersed particles. Up to this point the structure of a single-component laser doppler anemometer has been described. In the case of multi-component laser doppler anemometers for each direction, in which the speed of the dispersed particles is to be determined, two laser beams running in the respective direction are superimposed on each other in the measurement volume.
The invention relates to the adjustment of either a single-component or multi-component laser doppler anemometer.
In a known method of adjusting a laser doppler anemometer, laser light is coupled into the output of the receiving optic to determine the focal point of the receiving optic. This focal point establishes the measurement volume position on the optical axis of the receiving optic. In the measurement volume, an apertured diaphragm is set up with a diameter in the magnitude of the laser beams' minimal total radiation width. The emitting optic is then adjusted so that the emitted laser beams completely penetrate the aperture of the apertured diaphragm. In order to increase precision, apertured diaphragms with smaller aperture diameters can subsequently be used when the adjustment steps are repeated. In addition, the aperture of the apertured diaphragm can be projected on a surface with a microscopic objective attached to the surface in order to be able to more precisely check the position of the laser beams in the aperture of the apertured diaphragm.
The known method has a number of shortcomings. Since the precision of the laser beams' coincidence has to be evaluated subjectively, the quality of the adjustment is strongly dependent on the operator's experience. Many wind tunnel working sections in which laser doppler anemometers are frequently used are closed off with windows. When penetrating the windows, the laser beams are refracted, leading to a shift in the position of the measurement volume in the wind tunnel. The magnitude of this shift is dependent upon the wavelength of the laser light used, the thickness of the window and the angle at which the laser beams strike the window. Taking this refraction into consideration when adjusting the laser beam is difficult since it is not possible to bring the apertured diaphragm and the microscopic objective into the wind tunnel. Even if this is possible, flows in the working section when the wind tunnel is in operation can lead to a modification of the optical properties of the fluid used, in particular to a modification of its refraction index. This results in variations in the amount and/or the direction of refraction of the incident laser beams at the interface between the fluid and the windows, limiting the working section. In this way a shift of the crossing point of the laser beams with respect to the focal point of the receiving optic can occur, which then no longer coincide. An adjustment of the laser doppler anemometer during ongoing wind tunnel operations is not possible with the known methods so that, where there are modifications of fluid properties, speed measurements cannot be conducted. Since refraction at an interface between media with different optical properties is dependent upon the wavelength of the light used, a modification of the refraction index of the fluid flowing through a wind tunnel is particularly strongly felt in multi-component laser doppler anemometers which typically work with laser light of different wavelengths. The laser beams of the different wavelengths are affected by modifications in the refraction index with different intensity.